1. Field of the Invention
The present invention relates to an offshore drilling system for drilling wells under the bottom of the sea from the surface of the water in the deep sea, and also for a collection of natural resources such as manganese nodule, nickel, cobalt rich clast and the like on the bottom. The deep sea in the present invention referred to a sea the depth being not less than 1000 m.
2. Description of the Related Art
Hitherto, according to the conventional floating offshore drilling unit, an object of drilling is about 500 m or less in the depth of the water, a floating drilling marine structure is equipped with a fixed derrick, and lifting up and down of pipes are performed by a drilling unit in which a substructure (including a drill floor and a derrick) does not travel.
Under drilling in a great depth, a hoist for lifting up and down various pipes undergoes various different loads in compliance with individual works. The maximum load is the sum total of weight of a very long riser pipe due to a large depth, weight of a BOP (blow-out prevention device) and dynamic load caused by a vertical motion of the hull due to a wave, a wind and a wave motion. For example, in case of laout 2500 m of depth of the water, there will be produced a large load such as about 1000 tons in the total weight. Thus, it is indispensable to set up a hoist having a drawbar pull not less than 1000 tons.
On the other hand, it is considered that the suspension weight of drill strings, which will yield when a drilling is carried out, is about 400 tons, assuming that a drilling depth is 10,000 m. Therefore, in consideration of only the case of drilling, it is a difficult requirement to provide a 1000 tons class of hoist. Further, in the time of working of a lifting up-and-down pipe for drill strings frequently carried out, a device having a great drawbar pull involves such a problem that a hoisting speed is down on a physical basis in view of excessive pulley diameter, rope diameter, number of stages of rope , and weight, whereby the working efficiency will be extremely lowered.
In offshore oil drilling and scientific drilling, in view of special characteristics of work, in many cases, there is performed a coring (sampling of a geologic stratum) work or the like. This causes working hours for a lifting up-and-down pipe for drill strings to be increased. Thus, there is a need to reduce the whole working hours by effectively practicing those processes at higher speed.
Further, it is required to reduce a lead time in view of bad weather in a possibility of a typhoon or the like. Drill strings or the like inside a riser pipe is less in resistance and involves no fear of detention. Thus, the use of the drill strings or the like inside a riser pipe makes it possible to contribute to a higher speed as to a lifting up-and-down pipe at the time of passage through the riser pipe. In view of the foregoing, there is a need to make full use of this merit or advantage through high speed hoisting.
Hitherto, according to works of horizontal traveling and storage for a BOP on a drilling dredge or ship, it happens that a lifting-down load is not less than 150 tons. Consequently, there is a problem that this involves various dangers when the BOP is lifted up and horizontally traveled. Works on the drilling dredge involve various dangers such as a collision of the BOP with heavy objects due to pitching and/or rolling of the drilling dredge or due to the sway of the BOP, an accident of a fall to the sea, and works on a high scaffolding. It contributes to providing high efficiency of works, labor saving and improvement in safety that works of horizontal traveling and lifting up and down for the BOP system are mechanically performed.